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I use msysgit, and although it is still a preview release, I've had good luck with it. If you're looking for a stable release on windows you can either go with the officially supported cygwin install (it's not fun), or wait a little longer.

People have already mentioned TortoiseGit. Also, some IDEs have support for git, but it's going to depend on the type of development you're doing. I write Java code using IntelliJ Idea, which has a pretty decent support for git. Nothing fancy, but good enough for pushing and pulling code from by bitbucket account.

I second Tower (http://www.git-tower.com/). It's the best Git client for OSX by far! I use it for multiple clients and multiple hosting providers.

One thing to note is that it is advertised as SVN friendly, but I find that to be false. I don't use it for SVN, so it works out fine, but just do not purchase it with the intent of using SVN with it too.

Oh! And I find the support via Twitter (@gittower) to be better than filling out a report on their website. The second best is emailing them directly (support@git-tower.com). If all else fails, fill out the form and say a prayer!

It is a bit of money, but it is WELL worth it in my opinion. It is not yet available on Windows, but SmartGIT is the best (and only, to a certain extent) GUI that is available on OSX and Windows (and Linux, if thats your thing). And its free for personal, non-commercial use too.